The effects of hemorrhagic shock on the hearts of anesthetized dogs will be studied. Measurements will be made of cardiac output myocardial blood flow (radio-labelled microsphere method) and of myocardial O2 and lactate extraction before, during, and after a period of hemorrhagic hypotension. The heart and other organs (especially the intestines, liver, lung and kidney) will be examined anatomically. These findings will be compared with those made in dogs that are, in addition, treated with dipyridamole or verapamil. The hearts of patients who have died following a surgical operation utilizing a heart-lung bypass procedure will also be studied by light and electron microscopy.